An audiometer is used to test a subject's hearing over a range of frequencies and intensity levels. The audiometer creates an audio signal of a given frequency, intensity, and duration. The subject indicates to the test administrator if the audio signal was audible. The frequency range of the human ear is approximately 20 Hz to 20,000 Hz; however, it is typical that an audiometer can have a range of 125 Hz to 12,000 Hz. The human ear has a 120 db range, but the ear is not equally sensitive at all frequencies. This requires the electronics used to test hearing to have a somewhat larger dynamic range--135 db is typically required. This dynamic range exceeds that currently available from even the best amplifiers. The dynamic range of an amplifier is limited by its signal-to-noise ratio (SNR). The best SNR currently available is about 120 db. To get the dynamic range required, prior art audiometers insert a passive attenuator between the amplifier and the test earphone. The passive attenuator attenuates both the signal and the noise, thus allowing the audiometer to produce low-level signals at a reasonable SNR. Prior art audiometers switch these attenuators in or out using relays or solid-state switches. When the passive attenuator is switched into the circuit, the main attenuator is increased by the attenuation value of the passive attenuator. This increases the SNR at the earphone by the attenuation value of the passive attenuator. However, the main attenuator and the passive attenuator do not switch at the same rate (the main attenuator is faster). For a brief period of time, the output level will be too large. This produces the perception of a click, especially if the change is made while a signal is being presented. This problem is particularly severe at low power where the click produced by switching is comparable in power to the desired tone.
The undesired transient electronic clicking created by the switching of attenuators or pads in and out of the circuit is particularly troublesome in the von Bekesy test. In this test the subject depresses a button when the tone is audible, reducing the output power. When the power has been reduced to the point where the tone is inaudible the subject releases the button, increasing the power. When the tone is again audible, the subject again depresses the button, and the process repeats. Throughout the test the frequency is swept slowly from approximately 100 Hz to 10,000 Hz, and the maximum and minimum decibel levels at each frequency are recorded. The hearing threshold for a particular frequency is taken as the average of the maximum and minimum decibel levels.
A subject typically has a hearing threshold that increases with frequency. Thus, as frequency increases the subject eventually cannot hear the output, and must depress the button to increase the output power. To increase the power, attenuators must be switched, often creating an audible click having approximately the same power as the output tone. The test subject could interpret the audible click as part of the test, and release the button even though the tone is not audible. This results in inaccurate measuring of the hearing threshold, and could possibly result in the subject causing the power output to oscillate about the decibel level where attenuators are changed. Even if the subject recognizes that the click is not part of the test, it is still distracting and could cause error to be introduced into the test.